The present invention relates to apparatus and methods for chemical mechanical polishing a substrate.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface can present problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. In addition, plaranization is needed when polishing back a filler layer, e.g., when filling trenches in a dielectric layer with metal.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a polishing pad, e.g., a circular pad or linear belt, that moves relative to the substrate. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
During CMP operations, the polishing pad needs to be replaced periodically. For a fixed-abrasive pad, the substrate wears away the containment media to expose the embedded abrasive particles. Thus, the fixed-abrasive pad is gradually consumed by the polishing process. After a sufficient number of polishing runs the fixed-abrasive pad needs to be replaced. For a standard pad, the substrate thermally and mechanically damages the polishing pad and causes the pad""s surface to become smoother and less abrasive. Therefore, standard pads must be periodically xe2x80x9cconditionedxe2x80x9d to restore a roughened texture to their surface. After a sufficient number of conditioning operations, the conditioning process consumes the pad or the pad is unable to be properly conditioned. The pad must then be replaced.
One problem encountered in the CMP process is difficulty in replacing the polishing pad. The polishing pad may be attached to the platen surface with an adhesive. Significant physical effort is often required to peel the polishing pad away from the platen surface. The adhesive then must be removed from the platen surface by scraping and washing with a solvent. A new polishing pad can then be adhesively attached to the clean surface of the platen. While this is happening, the platen is not available for the polishing of substrates, resulting in a decrease in polishing throughput.
In one aspect, the invention is directed to a chemical mechanical polishing apparatus. The apparatus has a rotatable platen, a generally linear polishing sheet releasably secured to the platen to rotate with the platen, and a drive mechanism to incrementally advance the polishing sheet in a linear direction across the top surface of the platen. The polishing sheet has an exposed portion extending over a top surface of the platen for polishing the substrate, an unused portion wound around a feed roller, and a used portion wound around a take-up roller. The drive mechanism plays out a fixed length of the polishing sheet from the feed roller each time the drive mechanism advances the polishing sheet.
Implementations of the invention may include one or more of the following features. The drive mechanism may include a rigid frame that can be actuated to push the polishing sheet by a fixed distance. An enclosure may receive the feed roller, and the rigid frame may comprises a door to the enclosure. The door may be pivotally attached to the platen. The drive mechanism may include any of a pneumatic actuator to push the door by a fixed distance, a motor that applies a constant torque to the take-up roller, slip clutch that prevents the feed roller from rotating unless the polishing sheet is pulled with a first force that is greater than a second force applied by the motor, and a pneumatic actuator to push the rigid frame with a third force which is greater than the first force. The platen may have a channel to vacuum-chuck the polishing sheet to the platen.
In another aspect, the invention is directed to a method of operating a chemical mechanical polishing apparatus. In the method, a first portion of a generally linear polishing sheet is positioned to extend over a top surface of a rotatable platen. The polishing sheet includes a second portion wound around a feed roller and a third portion wound around a take-up roller. A rigid frame is actuated to push on the polishing sheet between the feed roller and the take-up roller, thereby generating slack in the polishing sheet, and a torque is applied to the take-up roller while holding the feed roller fixed to make the first portion of the polishing sheet taught.
Implementations of the invention may include one or more of the following features. At least a section of the first portion of the polishing sheet may be releasably secured to the platen before actuating the rigid frame. The rigid frame may push on a part of the polishing sheet located between the section of the polishing sheet secured to the platen and the feed roller, thereby generating slack in the part of the polishing sheet. The section of the polishing sheet may be released after actuating the rigid frame. The rigid frame may be actuated by a fixed distance.
In another aspect, the invention is directed to a method of chemical mechanical polishing in which a substrate is brought into contact with a generally linear polishing sheet that extends over a top surface of a rotatable platen. The polishing sheet includes an unused portion wound around a feed roller and a used portion wound around a take-up roller. The polishing sheet is releasably secured to the platen. The platen is rotated to rotate the polishing sheet and create relative motion between the substrate and the polishing sheet. The polishing sheet is released from the platen and incrementally advanced in a linear direction across the top surface of the platen by playing out a fixed length of the polishing sheet from the feed roller.
Potential advantages of the invention may include the following. A polishing sheet can be incrementally advanced by a repeatable spacing. The incremental advancing mechanism can installed in a rotatable platen.
Other features and advantages will be apparent from the following description, including the drawings and claims.